Enhancement of aluminum oxide physical vapor deposition with a secondary plasma

Reactive sputtering of aluminum oxide in a planar magnetron system is conducted with a mixture of O₂ and Ar reacting with and bombarding an aluminum target. The aluminum target is powered by a pulsed directed current (DC) bias which functions to discharge the accumulated ions on the insulating AlO_x film surface during the positive duty cycle and suppresses arc formation. A seven-turn helical antenna sits below the magnetron sputtering system in the vacuum system and delivers radio-frequency (RF) power to generate a secondary plasma in the chamber. This plasma can efficiently ionize the sputtered flux, achieving ionized physical vapor deposition (IPVD). A gridded energy analyzer (GEA) and a quartz crystal microbalance (QCM) are located in the substrate plane to allow the ion and neutral deposition rates to be determined. Electron temperature and electron density are measured by a RF compensated Langmuir probe. A RF power of 500 W significantly increases the deposition rate of AlO_x up to half of the Al deposition rate in metallic mode at the total pressure of 1.33 Pa (10 mtorr). At 3.33 Pa (25 mtorr), the ionization fraction of Al atoms reaches 90%. In addition the RF power extends the range of O₂ partial pressure in which the sputtering occurs in the metallic mode. SEM photos show that the secondary RF plasma makes the films smoother and denser due to a moderate level of ion bombardment. The deposition rates and ionization fractions fluctuate as a function of O₂ partial pressure. These variations can be explained by the combined variation of sputtering at the target, electron temperature and electron density.